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Mitosis/Transcript
Transcript Text reads: The Mysteries of Life with Tim & Moby An animation shows a boy, Tim, standing next to a robot, Moby. Moby has just finished making a sandwich. TIM: Hey, can I have half? MOBY: Beep. Moby breaks the sandwich into two parts. One part is clearly larger than the other. Moby eyes both parts of the sandwich, and offers the smaller part to Tim. Tim frowns and reads from a typed letter. TIM: Dear Tim & Moby, How do cells divide? From, Courtney. TIM: Well, people don't grow when making their body cells larger. Instead, cells divide, making two cells from one. And luckily, cell division is a lot more precise than Moby’s sandwich-dividing skills. Moby frowns. TIM: When a cell divides, the nucleus leads the way. An animation shows a cell. Its nucleus is in the center. The nucleus contains several thread-like objects. TIM: We call it mitosis when the nucleus divides into two identical nuclei. The nucleus of the cell splits into two. Then the rest of the cell divides. The result is that two cells are formed. TIM: This ensures that each cell offspring will have a complete set DNA. DNA contains all of the cell's instructions, written in a special chemical code. It's what controls a cell, kinda like its brain! The thread-like substances in the cell are DNA. An inset bubble shows a close-up image of DNA. It looks like a double-helix. TIM: Right before mitosis happens, cells make a copy of their DNA. A second copy of the DNA appears. TIM: During mitosis, those copies get pulled apart, with one ending up in each nucleus. The animation shows two cells after mitosis. Each cell has one copy of the DNA. MOBY: Beep? TIM: Let's break it down. For most of a cell's life, it's in a state called interphase, and its DNA is in a loose tangle. It's in this stage that cells prepare for mitosis by copying all of their DNA. So, even while they're doing their regular jobs, cells are getting ready to split. An animation shows a single cell copying its DNA. TIM: Mitosis itself has four steps: prophase, metaphase, anaphase, and telophase. An animation shows images representing each of the four phases of mitosis. TIM: During the prophase, the DNA is coiled up into X-shaped structures. These are called chromosomes and they form only when a cell is dividing. An animation shows chromosomes forming. The chromosomes are shaped like an X. TIM: Each X is actually two copies of the same DNA, connected at a point called the centromere. The animation reveals that the chromosome is one of four chromosomes in a nucleus. The center of each chromosome has a dot, which is the centromere. TIM: Then, little structures called centrosomes move to opposite ends of the cell. The centrosomes move to the left and right. TIM: They sprout thread-like spindles that stretch back toward the center. Many blue spindles reach out from each centrosome. TIM: In metaphase, the chromosomes line up along the center of the cell. The spindles latch onto the centromeres and tug. The four chromosomes arrange themselves into a vertical line. A spindle from each centrosome is attaches to each centromere. The spindles start to pull the centrosomes. TIM: This starts anaphase, when the identical halves of each chromosome get pulled apart to opposite sides of the cell. The spindles retract to the centrosomes they came from. They bring the chromosome halves with them. TIM: The final phase is called the telophase. The spindle fibers start to disappear, and each set of chromosomes develops a nuclear membrane, a thin sac that divides the nucleus from the rest of the cell. The animation shows the spindle fibers fading away. A membrane appears around each set of chromosomes. TIM: As this is happening, the chromosomes unwind. The chromosomes transform into thread-like DNA again. TIM: After mitosis, the cell membrane splits and two whole new cells are formed. The animation shows the cell membrane splitting. MOBY: Beep. TIM: That's how mitosis ends in animal cells, but plant cells do that last part a little differently. An animation shows a green plant cell. It has a rectangular shape. It also has a cell wall on its exterior instead of a membrane. TIM: They have rigid cell walls, so they build a new wall down the center instead of splitting. The animation shows a plant cell at the end of mitosis. It has two nuclei. A new cell wall appears between the two nuclei. MOBY: Beep. TIM: Well, living organisms grow by cell division, so mitosis is pretty important. An animation shows Moby putting on a construction hat and goggles. The hat has one mechanical arm with a laser and one mechanical arm with a ruler. TIM: Hey, um, what are you doing? The mechanical arm on Moby's hat holds the ruler up to a sandwich. TIM: Trying to make them perfectly even, huh? MOBY: Beep. TIM: Good, good. The laser beam starts to divide a sandwich. Tim slowly takes several of Moby's other sandwiches while Moby is distracted. TIM: Uh, let me know when you get it exactly right. Tim backs away, carrying an armful of sandwiches. Category:BrainPOP Transcripts Category:BrainPOP Science Transcripts